A) Field of the Invention
The present invention relates to a semiconductor device and its manufacture method, and more particularly to a semiconductor device having a so-called mushroom electrode and its manufacture method.
B) Description of the Related Art
The operation speed of a field effect transistor depends upon the gate length along the current path direction. In order to speed up a field effect transistor, it is desired to shorten the gate length. If the resistance of the gate electrode increases, a high speed operation of the transistor is restrained. In order to lower the gate electrode resistance, it is desired to set the cross sectional area of the gate electrode to a predetermined value or larger.
These requirements can be met by a mushroom type gate electrode which has a limited size of a lower part and a magnified size of the upper part. A generally upright lower part of the mushroom electrode is called a stem and the upper part with the magnified cross sectional area is called a head. A mushroom gate electrode is formed by vapor-depositing a gate electrode layer on a photoresist layer having a lower opening with vertical side walls and an upper expanded opening, and lifting off the resist layer.
As the aspect ratio of a vertical opening to be formed in a resist layer becomes large, it becomes difficult to uniformly bury the lower vertical opening with a gate electrode layer. In order to mitigate this difficulty, it has been proposed to form an upwardly broadening lower opening of a forward taper shape in a resist layer, and vacuum-deposit an upwardly broadening gate electrode stem of a forward taper-shape without forming any void.
In forming an upward broadening gate electrode stem of a forward taper shape, it is important to reliably control a gate length and a contact cross section between semiconductor and the gate electrode in order to improve the performance and reliability of the device. A conventional tapering method is, however, insufficient in that a uniform opening shape and a gate electrode cross-sectional shape at the contact area between semiconductor and the gate electrode cannot be formed reliably.
If a field effect transistor to be formed has a gate length longer than 0.15 μm, a mushroom gate electrode can be formed without any problem by forming a lower opening with generally vertical side walls in a photoresist layer. If a device having a gate length equal to or shorter than 0.15 μm is formed by a conventional method, a manufacture yield of gate electrodes lowers.
It is desired to form an upwardly broadening resist opening of a forward taper shape for forming the stem of the gate electrode.
In forming an upwardly broadening gate electrode of a forward taper shape by a conventional method, a gate electrode stem opening is formed in a resist layer and is forwardly tapered by utilizing glass transition. This conventional method has, however, poor controllability so that a uniform gate length is difficult to be set. Because of poor controllability, the cross section at the contact between semiconductor and the gate electrode is difficult to be controlled and an operation speed and reliability of devices cannot be improved.
A fine gate opening for a conventional mushroom gate having a high aspect ratio is upwardly broadened by utilizing resist glass transition. This method has, however, poor controllability and is difficult to obtain a uniform opening length, i.e., gate length. Because of poor controllability, it is difficult to control the cross section of the contact area between semiconductor and the gate electrode and improve the operation speed and reliability of devices.